The invention relates generally to steam turbines and more specifically to the arrangement of final stages of nozzle assemblies in the steam turbines with Singlet nozzle airfoils.
Steam turbines typically include static nozzle segments that direct the flow of steam into rotating buckets that are connected to a rotor. In steam turbines, the nozzle, including the airfoil construction, is typically called a nozzle assembly or diaphragm stage. Conventional diaphragm stages are constructed principally using one of two methods. A first method uses a band/ring construction wherein the airfoils are first welded between inner and outer bands extending circumferentially about 180 degrees. Those arcuate bands with welded airfoils are then assembled, i.e., welded between the inner and outer rings of the stator nozzle assembly of the turbine. The second method often consists of airfoils welded directly to inner and outer rings using a fillet weld at the interface. The latter method is typically used for larger airfoils where access for creating the weld is available and band construction is impractical
There are inherent limitations using the band/ring method of assembly. A principle limitation in the band/ring assembly method is the inherent weld distortion of the flowpath, i.e., between adjacent airfoils and the steam path sidewalls. The weld used for these assemblies is of considerable size and heat input. That is, the weld requires high heat input using a significant quantity of metal filler. Alternatively, the welds are very deep electron beam welds without filler metal. This material or heat input causes the flow path to distort e.g., material shrinkage causes the airfoils to bow out of their designed shaped in the flow path. In many cases, the airfoils require adjustment after welding and stress relief. The result of this steam path distortion is reduced stator efficiency. The surface profiles of the inner and outer bands can also change as a result of welding the nozzles into the stator assembly further causing an irregular flow path. The nozzles and bands thus generally bend and distort. This requires substantial finishing of the nozzle configuration to bring it into design criteria. In many cases, approximately 30% of the costs of the overall construction of the nozzle assembly is for restoration from the deformation of the nozzle assembly, after welding and stress relief, back to its design configuration.
Also, methods of assembly using single unshrouded airfoil construction welded into rings do not have determined weld depth, lack assembly alignment features on both the inner and outer ring, and also lack retention features in the event of a weld failure. These fillet-welded airfoils also have significant distortion issues as described previously for the band construction. Further, current nozzle assemblies and designs do not have common features between nozzle sizes that enable repeatable fixturing processes. That is, the nozzle assemblies do not have a feature common to all nozzle sizes for reference by machine control tools and without that feature each nozzle assembly size requires specific setup, preprocessing, and specific tooling with consequent increase costs. Accordingly, there has been demonstrated a need for an improved steam flowpath for a stator nozzle which includes low input heat welds to minimize or eliminate steam path distortion resultant from welding processes as well as to improve production and cycle costs by adding features that assist in assembly procedures, machining fixturing, facilitate alignment of the nozzle assembly in the stator and create a mechanical lock to prevent downstream movement of the nozzle assembly in the event of a weld failure.
The last few of stages of diaphragms in a steam turbine are typically called fillet fabrications (FF). The FF construction includes unshrouded airfoils (nozzles) welded to an outer and inner ring. The assembly is sometimes done with a costly and complex fixture, or done using scribed lines and no fixture. Both cases use a large weld fillet at the airfoil to sidewall interface to achieve required weld strength. One of the issues with this type of construction is the amount of weld distortion during fabrication. Another issue is the cycle time (labor hours) required to do the set-up, welding and possible area adjustments after fabrication. Most of these last stages are also very large and require lifting assistance due to the weight of the airfoils.
Accordingly, it would be desirable to provide such fillet construction diaphragms and techniques for fabricating such components with reduced cycle time and reduced weld distortion. Additionally, it would be desirable to improve turbine performance through improved airfoil tolerances and throat control.